1. Field of the Invention
The present invention relates to a distortion compensating apparatus, for example, to a distortion compensating apparatus, which may be suitably adapted to an amplifier for transmission by a radio base station used in the mobile communication system.
2. Description of the Related Art
A radio base station in the mobile communication system is provided with a distortion compensating apparatus to reduce distortion generated in a power amplifier in order to realize large power transmission to mobile stations.
Various systems have been proposed as the distortion compensating system. The predistortion system is an example of such distortion compensating system.
The predistortion system has been proposed to reduce distortion of the signal after amplification thereof by applying an inverse distortion characteristic to the distortion generated in the amplifier to the signal before input to the amplifier.
Here, since distortion generated in the amplifier changes depending on the input signal (particularly, the level of the input signal), it is preferable that the coefficient (distortion compensation coefficient) for applying the inverse distortion characteristic is calculated as a function of the input signal and the input signal is compensated based on this coefficient.
Therefore, a coefficient table (Look-Up Table) for storing the distortion compensation coefficient corresponding to the address generated from the input signal is used in the predistortion system.
The Look-Up Table (LUT) is triggered by activation and reset of the power supply to quickly write the coefficients with an exclusive circuit and to complete the setting of all coefficients before starting the operation.
The predistortion system is disclosed in the Japanese Unexamined Patent Publication No. 2001-267850.
According to the background of the invention described above, coefficients are written at a high speed with an exclusive circuit into the look-up table in the distortion compensating apparatus using the start and reset of power supply as the trigger.
However, precise distortion compensation requires storing of distortion compensation coefficients for small changes of the input signal. As a result, the number of coefficient data to be written increases and a longer time is therefore required until completion of the data writing. Accordingly, a longer time is required until the amplifier can start from the start of operation.